Self-monitoring trigger generator



Dec. 7, 1954 J. w. GRAY SELF-MONITORING TRIGGER GENERATOR Filed Nov. 16, 1949 2 Sheets-Sheet 1 n52 GEEHBR TIME 3nventor I JOHN W GHQ)" attorney 54 J. w. GRAY SELF-MONITORING TRIGGER GENERATOR ecl 2 Sheets-Sheet 2 Filed Nov. 16, 1949 T/ME P Un d States Patent v 2,696,557 SELF-MONITORING TRIGGER GENERATOR John W. Gray, White Plains, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application November 16, 1949, Serial No. 127,731

8 Claims. (Cl. 25027) This invention relates to a self-monitoring trigger generator adapted to produce a symmetrical rectangular output despite wide variation in amplitude of a sinusoidal input.

Such devices are particularly useful in systems where itis required to count the successive axis crossings of an input signal whose amplitude may vary over a wide range during the period of time of such count.

Devices of this nature as heretofore proposed have sufiered from a number of disadvantages which seriously limits their utility for these purposes, among which are the production of outputs which are not truly rectangular, overshoot at each reversal and lack symmetry of output.

If a trigger generator is to produce a rectangular output wave whose cyclic operation is to coincide with the instants of time when an input wave crosses a chosen axis, it is necessary that the axis of operation of the generator, that is the median of the critical turn-on and turn-off potentials of the tubes used, coincide with the chosen axis of the input signal. Otherwise the output will not be symmetrical and some alternations of low amplitude may not be counted at all.

In prior devices this loss of symmetry and actual miscount has been mitigated only by careful match of the tubes used necessitating careful rematch on each replacement and even with such precautions improper operation has been liable to occur as the characteristics of the tubes changed during their use.

The instant invention avoids these former difficulties by providing a circuit wherein the trigger generator itself acts as a self-correcting means always readjusting itself so that the axis about which its operation takes place is automatically made to coincide with the axis about which the input signal alternates.

Thus the tubes used need not be carefully matched either on installation or replacement and any changes undergone thereby during their operational life do not afiect the accuracy of operation of the device.

In general, these advantageous results are obtained by making the direct current level of the grid of one of the tubes used depend on the average potential of the plate of that tube so that if the output tends to become asymmetrical the average potential of the grid is raised or lowered as the case may be returning the crictical operating levels to symmetrical conditions.

Likewise, a regenerative connection is utilized between the two tubes forming the trigger generator which is so adjusted that there is a sudden and instantaneous switching of plate current between the two tubes resulting in almost perfectly rectangular waves having no overshoot or other improper operation.

Keeping these advantages in mind, the purpose of this invention is the provision of a trigger generator for accurately counting all axis crossings made by a potential wave of varying potential and wave shape, and in such a generator the self-correction of its operating level to coincide at all times with the mean input signal level. i This invention will be more readily understood from the following detailed description in connection with the attached drawings, in which:

Figure l is a schematic illustration of one form of the invention.

-;Figure '2 shows graphically examples of typical input and output voltage wave shapes of the circuit of this invention.

Figure 3 graphically depicts the potential changes at several points within the circuit.

Figure 4 graphically depicts the manner in which the circuit automatically corrects aberrations.

Referring now to the drawings an input potential which may comprise an alternating wave of random amplitude as indicated by the curve 40 in Fig. 2 is applied to the conductor 11 of Fig. 1. This conductor is connected through a condenser 12 and resistor 13 to a terminal 36 which constitutes the junction of two diodes 17 and 18 connected in series across resistor 21 and 22.

Resistors 21 and 22 together with resistors 19 and 23 form a potential divider connected between a source of positive potential indicated diagrammatically as -|-B and ground, and the resistors 21 and 22 are so adjusted with respect to the remainder of the divider and to each other as to provide a potential drop of 2 volts across each. Thus any instantaneous potential impressed on the junction 36 which is greater than 2 volts in either a positive or negative direction will be transmitted through one or the other of the diodes 17 or 18, resistor 21 or 22 and a large condenser 42 connected between ground and the junction 41 of resistors 21 and 22 which junction is thus grounded as regards alternating current.

The diodes 17 and 18 therefore act as clipping diodes or limiters never permitting the junction 36 to vary by a greater extent than :2 volts. This action is illustrated by the curves of Fig. 2 wherein the horizontal lines 37 and 38 represent the upper and lower two-volt limits and the dotted portion of the curve 40 those alternations which exceed these limits and which, therefore, are clipped by the diodes so that the potential of the junction 36 is constrained to vary only as the solid portions of the curve 40 including the truncated or flattened portions 45.

Those potentials which exceed the limits set by the potential drop of resistors 21 and 22, namely the dotted portions of the curve 40, result in current flow through resistor 13 which charges or discharges condenser 12 depending on the sense of such current flow. The average of these charging and discharging currents, however, must necessarily be zero over a time interval determined by the time constant of the condenser 12 and resistor 13. Thus, by making this time constant sufficiently long to include a number of cycles of the input signal, the input signal is positioned so that the areas included between the upper dotted portions of the curve 40 and the line 37 are equal to the areas included between the lower dotted portions of the curve 40 and the line 38, for that number of cycles covered by the time constant. The clipping action of the diodes 17 and 18, therefore, serves not only to limit the amount of excursion in potential of the junction 36 but also to center those excursions about the axis 39 making the signal at the junction 36 symmetrical about such axis, which may, for example, have a static potential of volts.

This signal, thus clipped and symmetrically positioned as respects a chosen voltage axis, is impressed on the grid 14 of a tube 16 which, together with the tube 43, constitutes the rectangular wave generator.

These two tubes have a common cathode resistor 44, so that if a positive signal on the grid 14 of the tube 16 causes current flow in that tube, the cathode feedback coupling constituted by the resistor 44 tends to cut off current flow in the other tube. Also, the plate 46 of the tube 16 is connected through a small condenser 47 with the grid 48 of the tube 43, constituting a second feedback link which produces diiierential tube action. For instance, if a positive signal on the grid 14 causes an increase of current in tube 16, the resulting potential drop in the resistors 57 and 58 is communicated as a negative step through the condenser 47 to the grid 48 of the tube 43, tending to cut off the current therein. One of the plate resistors 57 is adjustable so that the intensity of this regenerative action can be manually controlled thereby adjusting the circuit for best sensitivity and stability at a given input signal frequency.

An output connection is made to the generator at the plate 53 of tube 43 as is indicated in Fig. 1 by the conductor 56. As before stated, the action of the two tubes 16 and 43 is differential because of the action of the two feedback links connecting them. That is, when one conducts current the other is cut off, and when the first stops conducting the other tube begins to conduct. As a result, a positive signal on the grid 14 causes an increase in the potential of the output conductor 56, and a negative input signal causes a negative output potential step. A perfectly rectangular output potential wave such as is depicted by graph 66 in Fig. 2 is thus generated by any random input potential wave such as that of the graph 40.

The grid 48 of the tube 43 is returned to ground through a resistor 49 connected to the intermediate terminal 61 of a potential divider consisting of the resistors 51 and 52 in series. Resistor 52 being connected to ground and resistor 51 connected to the anode 53, the average operating potential of the grid 48 is thus made proportional to the average operating potential of the anode 53. This method of grid return resulting in control of the operating levels of the two tubes by the average output level has a profound effect in improving the accuracy of the generator as a counting device, as will be eX- plained more fully hereinafter. In addition, this method of grid return, since it constitutes a degenerative feedback connection, very nearly completely eliminates effects of tube and other component changes during life and eliminates any necessity for matching tubes.

The graphs of Fig. 3 depict the potential conditions at several points in the circuit during a complete cycle. Reference character 67 indicates the instant when the tube 16 begins to conduct due to increase of the input signal to an above-average value of potential. This change of the input signal to a positive value is indicated by the potential graph G1, representing the potential of the grid 14, which rises above its average axis 62 at the time 67. This increase in the grid potential starts current flow in the plate circuit of the tube, resulting in a potential drop in the plate resistors 57 and 58 and a corresponding reduction in potential of the anode 46. This negative potential step is transmitted through the condenser 47 to the grid 48 of the tube 43, and makes that grid suficiently negative to stop current flow in that tube. The resulting reduction of cathode 63 potential is communicated to the cathode 59 because the two cathodes are connected directly together and this starts full current flowing in the tube 16. The resulting reduction of plate 46 potential is shown in Fig. 3 as a lowering of the graph P1. The graph K depicts the lowering of the cathode potential which sets itself above the potential G1 of the grid 14 by the amount required in the type of tube employed to maintain current in the cathode resistor. For instance, if the 6J6 type of tube be employed, a grid bias of 3 /2 volts will maintain a current of 3 milliamperes. The reduction of the potential of the grid 48 at time 67 is depicted by G2, and P2 indicates that the full +B voltage exists at the anode 53 of the tube 43,

since this tube conducts no current between time 67 and time 68.

Because of the actions completed at the time 67, the condenser 47 thereafter is in the process of being charged through the resistor 49 to the potential of the junction 61. This asymptote potential may conveniently be about 116 volts and is, of course, determined by the relative sizes of the resistances 51, 52 and 54. However, the time constant of the RC circuit including the capacitor 47 and the resistors 49, 51, 52 and 54 is made large relative to the incoming signal period, so that the potential G2 of the grid 48 does not rise appreciably during the short period of a half cycle depicted in Fig. 3 between time 67 and 68.

At the time 68 the incoming signal reverses and becomes negative, placing a negative potential step G1 on the grid 14. This reduces the current flow in the tube 16, increasing the potential of its anode 46 and creating a positive potential step through the condenser 47 on the grid 48. This starts some current flow in the tube 43, which reacts through the cathodes to help stop current flow in the tube 16. These actions aiding each other and being cumulative, result in an almost instantaneous switching of the current from one tube to the other, even though started by only a small input signal potential change. When the tube 43 carries current, its anode 53 and the output conductor 56 have a lower potential, say, 180 volts, and the junction 61 therefore determines an asymptote potential which may be, for instance, 84-volts, toward which level the condenser 47 4 discharges. At this time the increased potential of the grid 48 is represented by the graph G2 in Fig. 3 between times 68 and 69, and the cathode potential K still maintains itself approximately 3 /2 volts above G2.

At the time 69 the input signal again becomes positive and the actions which occurred at time 67 are repeated.

As heretofore described, when input potentials greater than the clipping limits are employed, they are clipped and at the same time the input wave form is by the action of the clipping diodes, shifted up or down so that its mean potential is exactly midway between the two clipping potentials. However, when the input wave forms have a peak-to-peak value less than the potential between two clipping limits such as 37 and 38 in Fig. 2, the diodes do not serve to center the signal, and the alternations might drift up or down in average potential within the clipping limits so that their median axis would not coincide with the operating axis of the trigger generator were it not for an automatic corrective action in which the signals are centered by the action of the triode tubes 16 and 43. The same corrective action also occurs if, because of unequal thermal drift or any other cause, the resistance drops in any of the resistors 19, 21, 22 or 23 should undergo a relative change, causing a shift in the diode clipping limits relative to the operating level of the tubes 16 and 43. Any change in the characteristics of the tube 16 or 43 due to tube drift or tube replacement, and any change in resistors 51 and 52 will also be automatically corrected.

This corrective action occurs as follows. In Fig. 4 the operating potential level of the triodes is represented by the line 72. An input signal has a wave form '73 about a median potential 74 which dilfers from the potential 72. Consequently, the operating points where the graph 73 crosses the grid operating potential level 72 are not equally spaced in time and the plate potential therefore will dwell on one level longer than on the other. This potential is represented by the graph P2, in which the areas above and below the peak-to-peak median potential 76 are unequal and the time average potential is 77, being unequal to the median potential. However, it is the time average plate potential which determines the potential of operation of the grid 48 because of the feedback through resistors 49 and 51, and which in turn determines the potential of the cathode 63. The latter is also the operating potential of the cathode 59. Therefore, in the case shown in Fig. 4, the increase of the average plate potential 77 increases the average potential of the grid 48 and thus its potential axis of operation. This, in turn, raises the operating potential of the cathodes 63 and 59, and that in turn raises the average potential of the grid 14 represented in Fig. 4 at 72, which is tantamount to lowering the entire signal graph 73. This corrective action reduces the width of the upper half-cycle of P2 in Fig. 4, and increases the width of the lower half-cycles,

and it continues until the axis 72 is made to coincidewith the line 74 and the positive and negative half-cycles of P2 are equal in duration.

This automatic corrective action makes this trigger generator to a large extent self-monitoring and self-correcting with respect to interval variations of components and also with respect to external changes in the average axis of small input signals.

The design of this circuit prevents the grids 14 and 48 from drawing current at any time during the cycle, so that the potentials P1 and P2 never overshoot. This contributes to the production of .an excellentrectangular output wave shape.

The changes of potential levels in Fig. 3 are shown by slightly oblique lines for clarity. However, the time required to transfer current from one tube to the other is exceedingly short, so that at the scale drawn these lines would actually be vertical.

It is, of course, obvious to those skilled in the electronic art that in place of the triode tubes disclosed, tetrodes or pentodes may be employed with minor appropriate changes in the circuit, and that the circuit can be designed to operate over any desired range of frequencies at which the electronic tubes employed can operate.

What is claimed is:

l. A device of the character described comprising, first and second discharge tubes each having at least a cathode, anode and control grid, an input circuit connected to the control grid of said first tube, a resistor common to the cathode circuits of each of said tubes, a regenerative coupling between the anode of said first tube and the control grid of said second tube, an anode resistor connected between the anode of said second tube and a source of positive potential a voltage dividing network connected between the anode terminal of said anode resistor and the terminal of said common cathode resistor remote from said cathodes and a direct current connection between the control grid of said second discharge tube and an intermediate terminal of said dividing network.

2. A device of the character described comprising, first and second discharge tubes each having at least a cathode, anode and control grid, an input circuit connected to the control grid of said first tube, a common cathode resistor for said tubes, a condenser connected between the anode of said first tube and the control grid of said second tube, an anode resistor connected between the anode of said second tube and a source of positive potential a voltage dividing network connected between the anode terminal of said anode resistor and the terminal of said common cathode resistor remote from said cathodes, and a direct current connection between the control grid of said second discharge tube and an intermediate terminal of said dividing network.

3. A device of the character described comprising, first and second discharge tubes each having at least a cathode, anode and control grid, an input circuit connected to the control grid of said first tube, a common cathode resistor for said tubes, a condenser connecting the anode of said first tube to the control grid of said second tube, an anode resistor connected between the anode of said second tube and a source of positive potential a voltage dividing network connected between the anode terminal of said anode resistor and the terminal of said common cathode resistor remote from said cathodes, and a resistor connecting the control grid of said second discharge tube to an intermediate terminal of said dividing network.

4. A device of the character described comprising first and second discharge tubes each having at least an anode, cathode and control grid, an input circuit connected to the grid of said first tube, a regenerative circuit connecting said first and second tubes, an anode resistor connected between the anode of said second tube and a source of positive potential a voltage dividing network connected between the anode terminal of said anode resistor and ground and a direct current connection between the grid of said second tube and an intermediate terminal of said voltage dividing network.

5. A device of the character described comprising, a peak-clipping limiter having an output terminal, a rectangular wave generator comprising an input terminal and first and second discharge devices each having at least a cathode, anode and control grid, a common unbypassed cathode resistor comprising a cathode regenerative feedback connection, a condenser connected between the anode of the first said discharge device and the control grid of the second discharge device comprising a grid-plate regenerative feedback connection, an anode resistor connected between the anode of said second discharge device and a source of positive potential 3. potential divider having one terminal connected to the anode terminal of said anode resistor having a second terminal connected to the control grid of the second discharge device and having its third terminal connected to ground comprising a grid-plate degenerative feedback connection, and an electrical connection from the output terminal of said peak-clipping limiter to the input terminal of said rectangular wave generator.

6. A device of the class described comprising, a source of direct current potential having a resistance network connected in shunt thereto, a pair of diodes connected in series across a portion of said network, an input terminal, a series condenser and resistor connected between said input terminal and the juncture of said pair of diodes, a trigger generator including first and second tube sections each having at least an anode, cathode and control grid, said cathodes being connected to said direct current source through a common resistor, a condenser connecting the anode of said first tube section to the control grid of said second tube section, a voltage dividing network connected between the anode of said second tube section and said direct current source, a direct current connection between the control grid of said second tube section and an intermediate terminal of said voltage dividing network and a circuit connecting the junction of said pair of diodes and the control grid of said first tube section.

7. A device of the class described comprising, a source of direct current potential having a resistance network connected in shunt thereto, a pair of diodes connected in series across an intermediate portion of said network, a large capacitance connected between the midpoint of said intermediate portion and the low voltage terminal of said network, an input terminal, a series condenser and resistance connected between said input terminal and the juncture of said pair of diodes, a trigger generator including first and second tube sections each having at least an anode, cathode and control grid, said cathodes being connected to said direct current source through a common resistor, a condenser connecting the anode of said first tube section to the control grid of said second tube section, a voltage dividing network connected between the anode of said second tube section and said direct current source, a direct current connection between the control grid of said second tube section and an intermediate terminal of said voltage dividing network and a circuit connecting the junction of said pair of diodes and the control grid of said first tube section.

8. A device of the class described comprising, a source of direct current potential having a resistance network connected in shunt thereto, a pair of diodes connected in series across an intermediate portion of said network, a large capacitance connected between the midpoint of said ntermediate portion and the low voltage terminal of said network, an input terminal, a series condenser and res stance connected between said input terminal and the untcure of said pair of diodes, a trigger generator including first and second tubes each having at least an anode, cathode and control grid, said cathodes being connected to said direct current source through a common resistor, a condenser connecting the anode of said first tube to the control grid of said second tube, a voltage dividing network connected between the anode of said second tube and said direct current source, a resistor connecting the control grid of said second tube to an intermediate terminal of said voltage dividing network and a circuit connecting the junction of said pair of diodes and the control grid of said first tube.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Electronic Engineering, September 1947, page 282, Electronic Switching.

Terman: Radio Engineering,

a e 322, Fi s. 6-51, published 1947 by McGraw-Hill p g g Book Co., Inc. 

